One of the most important indexes of power systems is safe, steady operation. Therefore, a lot of expensive computers and communication equipment have been invested to set up scheduling and controlling systems having functions such as remote signaling, remote telemetering, remote controlling, remote tuning and safety monitoring of electric power systems. However, since an electric power system covers vast in territory, phase angle measurements can not be attained in real-time and directly. Phase angles are computed from power flow analysis or state estimations which are based on telemetering or voltages, active powers and reactive powers. This measurement method is not only inaccurate but also time consuming, and impossible to be used in safety and steady analysis or emergency control in electric power systems. Monitoring of power system stability has to be based on the change of active and reactive power in networks and steady state of power system be judged indirectly.
With development of science and technology, especially of computers, communication and satellite technology, it turns vast into near in territory to make phase angle measurements possible. There are, more, microwave and optical fiber communication with high speed and great capacity, and the global position system (GPS) having accurate location with .+-.15 m and accurate time within .+-.1 .mu.s. These advanced communication means and technology provide pledge for this invention.